Bridge circuits are typically used to drive bidirectional motors. FIG. 1 shows a conventional output buffering H-bridge circuit composed of four N-channel MOS transistors 2, 4, 6, and 8. The first output terminal 22 is connected to the second output terminal 24 through an inductor load 20. Control circuit 10 has a terminal for the power source 26, a ground terminal 28 and an input terminal 30. Control circuit feeds input terminals 12, 14, 16, and 18.
In most prior art H-bridge circuits, input terminals 12 and 18 are tied together and additionally input terminals 14 and 16 are tied together. When there is a high voltage level on 12 and 18, the first N-channel MOS transistor 2 and the fourth N-channel MOS transistor 8 becomes conductive. As a result the potential of output terminal 22 approaches about Vbat (supply voltage from battery) while the potential of output terminal 24 approaches 0V. Conversely, when the potential of the internal input terminals 14 and 16 go high, the second N-channel MOS transistor 6 and the third N-channel MOS transistor 4 become conductive. As a result the potential of the first output terminal 22 approaches about 0V while the potential of the second output terminal 24 approaches about Vbat.
In FIG. 1, the turn-off switching speed of the MOS transistors are slow compared with the turn-on switching speed of the MOS transistors. Therefore, MOS transistor 2 on the power source side and MOS transistor 4 on the ground side both become conductive for a short period of time and a shoot-though current flows from Vbat terminal 26 to ground terminal 28 through MOS transistors 2 and 4. When the shoot-through current flows in the H-bridge circuit the amount of consumed electricity in the H-bridge circuit increases and an undesirable large current spike is produced at power terminal 26 and ground terminal 28 which can cause system noise. Another disadvantage of shoot-through current is increased radio frequency interference which causes additional thermal dissipation in the output structure and loading on the power supply.
A solution for reducing shoot-through current is shown in U.S. Pat. No. 5,057,720 illustrated in FIG. 2. In this circuit there are two additional MOS transistors 14 and 15 which are connected to the H-bridge, which is comprised of MOS transistors 5, 6, 7 and 8. MOS transistor 8 is connected to the H-bridge in a manner that allows quick discharge from transistor 5 through transistor 14 when transistor 8 becomes nonconductive. Conversely, when the transistor 6 becomes nonconductive, transistor 15 becomes conductive resulting in a quick discharge from the gate of transistor 7 through transistor 15.
With the prior art configuration of FIG. 2, the amount of shoot-through current becomes smaller due to reduced turn-off time. However, several disadvantages still exist. One disadvantage of this configuration is that it requires a full H-bridge circuit; it cannot be used with a half H-bridge circuit. Another disadvantage is the required sensing of the turn-on of an opposite phase. The circuit assumes that transistors 5 and 8 are being turned on while transistors 6 and 7 are being turned off or vice versa. This circuit does not accommodate situations where the transistors 5 and 6 are being turned on while transistors 7 and 8 are being turned off or vice versa.